The present invention relates to an improved extrusion die whose actual die gap is continuously measured while polymeric melt is flowing through the die. More particularly, the present invention is directed to an adjustable gap extrusion die containing high temperature resistant proximity sensors located within the die.
Normally, in an apparatus for producing plastic sheet-like material such as plastic film, as shown in FIG. 1, a die having a plurality of bolts 10 for controlling the gap between the die lips is employed. A typical die gap is 80 mils (0.080 inch) with an adjustment range of plus or minus 10 mils. A molten plastic, for example, poly(ethylene terephthalate), is introduced into the die through an inlet 20 by a pump (not shown), extruded through a gap between a pair of die lips 13A and 13B, and is solidified to form a sheet-like material. The thickness of the plastic sheet produced by the above-described process is greatly influenced by the gap between lips 13A and 13B. More particularly, the distribution or pattern of thickness in the transverse direction of the sheet is primarily determined by the distribution of the gap between the die lips across the width of the die, hereinafter referred to as "die gap distribution."
Die gap distribution is controlled by rotation of adjustment bolts 10, thereby selectively pressing or pulling die lips 13A and 13B together or apart. However, because the die lips are typically made of one piece of monolithic metal, it is impossible to adjust one bolt precisely to the desired die gap thickness without altering the die gap thickness in the region of adjacent adjustment bolts 10. Because of this interaction it is extremely difficult to adjust die lips 13A and 13B to achieve the desired die distribution, even before the die is put into operation. The problem of accurate die gap adjustment is exacerbated by the non-linear performance of adjusting bolts 10, which display a "dead zone" and a "non-linear zone" in which adjustment of the bolt produces little or no change in the die gap thickness. These zones of non-linear performance, caused by thread tolerances and wear, are typically between one-half and a full bolt rotation.
With sufficient patience and time, an experienced worker can achieve the desired initial die gap distribution by a trial and error approach, first adjusting an adjustment bolt 10, then measuring the actual die gap distribution, readjusting the same or a different adjustment bolt 10, measuring the resulting die gap distribution, and so on until an acceptable die gap distribution is achieved. This procedure is often performed after the die has been heated to operating temperature (typically from 275.degree. to 325.degree. C.) to take into account any thermal effects upon die gap distribution.
Unfortunately, the desired die gap distribution may change during operation of the die i.e. with polymeric melt flowing through the die gap. Five to ten percent variation in die gap distribution during die operation is not uncommon.
The prior art has generally attempted to solve this problem by measuring the thickness of the finished plastic sheet or film across its width downstream from the die lips, hereinafter called "film thickness distribution," readjusting one or more of adjustment bolts 10, remeasuring the film thickness distribution, and so on until the film thickness distribution was once again within acceptable limits. T. Akatsuka et al., "Apparatus For Controlling The Gap Between Lips Of A Die For Extruding A Plastic Sheet Material," U.S. Pat. No. 4,124,342 (Nov. 7, 1978) discloses an automated system for control of film thickness distribution which employs an algorithm to calculate the number of turns each adjustment bolt 10 requires to achieve a desired die gap distribution and therefore the desired film thickness distribution. One of the disadvantages of such a system is the assumption that each die bolt response is uniform.
There have been several improvements in control of die gap adjustment as opposed to actual die gap measurement.
R. Mules, "Apparatus For Making Polymeric Film," U.S. Pat. No. 3,920,365 (Nov. 18, 1975) discloses control of film thickness distribution by selective thermal control of isolated or localized portions of die lips 13A and 13B by employing temperature sensors and heating elements embedded therein. By controlling localized temperature variations, the local melt viscosity, and hence local mass flow rate, of the polymeric material may be increased or decreased to maintain the film thickness distribution within acceptable limits.
R. Lowey, Jr., "Plastic Extrusion Die," U.S. Pat. No. 2,938,201 (May 31, 1960) discloses an adjustable sheet forming extrusion die having expandable adjustment bolts which may be finely adjusted by means of electric heaters which control the length of each bolt between its mounting in the die body, and the bolt juncture points in the die blades.
P. Farmer et al., "Extrusion Die Blade," U.S. Pat. No. 4,252,519 (Feb. 24, 1981) discloses replacement of the expandable adjustment bolts of Lowey with a thermally adjustable die blade with a heat barrier to minimize the effect of temperature variations on the temperature of the die blade lip.
P. Peterson et al., "Apparatus For Measuring Variations In Thickness Of Elongated Samples Of Thin Plastic Film," U.S. Pat. No. 3,764,899 (Oct. 9, 1973) discloses the use of a small area capacitance-effect electrode which is capable of measuring variations in the thickness of a dielectric material such as plastic film.
Die gap measurement and adjustment problems are not limited to sheet-forming extrusion dies, but rather may be encountered with adjustable dies of virtually any configuration.
P. Reitemeyer et al., "Apparatus For The Extrusion Of Tubular Bodies Of Synthetic Resin Material," U.S. Pat. No. 4,137,028 (Jan. 30, 1979) discloses an extrusion die for forming tubular plastic materials which employs a radioactive source located centrally within the tube and plurality of detector means located on the outside calibration sleeve. The radioactivity measurements so obtained are employed to maintain the inner or mandrel die in a centered position, thereby assuring a uniform thickness of the tubular material being extruded.